The present invention relates generally to steel alloys containing chromium which are coated with aluminum which may contain up to 15% silicon. More particularly, the present invention relates to an aluminized ferritic stainless steel, such as AISI Type 409. Continuous coating lines for hot dip aluminizing strip include in-line cleaning of surface oxides and annealing treatments. Many of these coating lines use a direct fired furnace at elevated temperatures with an atmosphere of gaseous products of combustion of fuel and air but no free oxygen. Strip is then normally heated in a radiant tube furnace and cooled to bath temperature. The strip enters the coating bath and the amount of coating metal is adjusted in a finishing operation.
Steel alloys containing chromium are known to be difficult to aluminize. This has generally been attributed to chromium oxides on the surfaces being very difficult to wet. Depending on the base metal composition, oxides of chromium, aluminum, titanium and silicon form during heat treatment and are not easily reduced. They remained on the surfaces of the steel alloy and inhibited the reaction between the substrate and the aluminum coating metal during the immersion of the strip in the bath. Uncoated portions and pinholes resulted.
Preparation of chromium alloy steel strip for hot dip aluminizing has included the cleaning of the strip and the maintaining of a protective hydrogen atmosphere prior to coating. Typically, the coating furnace was used to anneal the strip to develop the desired mechanical properties and bring the strip to a temperature above the bath temperature prior to coating. Various coating methods have been developed to improve the wettability of the chromium bearing alloys.
U.S. Pat. No. 4,891,274 teaches that silicon greater than 0.1% caused wettability problems and titanium greater than 0.16% acted as a reducing agent during steel melting and contributed to silicon being introduced to the melt from the slag and refractofies. Silicon levels below 0.1% were important for wettability to avoid the formation of silicon oxides on the strip during the coating process.
U.S. Pat. No. 4,675,214 taught that it was necessary to provide a reducing atmosphere once the strip exited the direct fired furnace to minimize chromium oxidation. Typically, the strip was heated from 677.degree. C. to 954.degree. C. in the radiant tube furnace having an atmosphere such as 20% by volume hydrogen with 80% by volume nitrogen and cooled to 660.degree. C. to 732.degree. C. in an atmosphere with almost pure hydrogen and a dew point preferably below -12.degree. C. and oxygen below 40 ppm before entering the coating bath.
U.S. Pat. No. 5,023,113 believed that even no free oxygen in a direct fired furnace still had a significant oxidizing potential due to the presence of water and the chromium present on the surfaces. Chromium oxide formed on the surfaces of the strip was not removed by the protective hydrogen atmosphere prior to entry into the coating bath. The temperature in the direct fired furnace was lowered while still removing the oil, din and iron oxide on the surfaces and attempted to avoid excessive oxidation of the chromium. The strip was then further heated to a fully annealed condition in another furnace section having at least 95% hydrogen, less than 200 ppm oxygen and a dew point less than 0.degree. F. (-18.degree. C.). The strip was then passed through the snout of the furnace having a protective atmosphere with at least 97% hydrogen and a dew point no greater than -20.degree. F. (-29.degree. C.) before passing into the coating bath.
U.S. Pat. No. 4,883,723 heated a ferritic alloy to a temperature of at least 1232.degree. F. (666.degree. C.) or the temperature of the molten aluminum bath. The atmosphere was at least 95% hydrogen and the dew point was no more than 40.degree. F. (5.degree. C.). The heating was typically done in a direct rued furnace and a radiant tube furnace which were connected to the coating bath.
Other approaches to improve the wettability of ferrous alloys containing chromium provided an intermediate coating prior to aluminizing. These coating layers were nickel or copper based or developed an iron-boron or iron-phosphorus layer prior to aluminizing. U.S. Pat. No. 4,891,274 provided a nickel coating to improve the wettability of chromium alloy steels. The patent taught that a satisfactory aluminum coating can not be obtained using conventional coating practices if the oxygen in the atmosphere is greater than 1 ppm and the dew point is higher than -40.degree. C. Control of these levels in the furnace was taught to be difficult and that the surfaces will suffer from oxidation with resulting poor wettability and coating defects.
Recently, there have been two other approaches to improve the wettability of chromium alloys for aluminum. The first one was EP 467,749 which taught a method which avoided the need for high purity hydrogen in the aluminizing furnace. By preheating the strip at less than 500.degree. C. (932.degree. F.) in a nonoxidizing atmosphere containing less than 3% oxygen and heating the strip in a second nonoxidizing atmosphere to a temperature less than 950.degree. C. (1740.degree. F.) in an atmosphere having a dew point of less than -40.degree. C. (-40.degree. F.) and preferably less than -50.degree. C. (-58.degree. F.), the atmosphere in the cooling furnace and snout did not need to be pure hydrogen. The strip surfaces could be passed through a nonreactive atmosphere such as nitrogen or a nitrogen/hydrogen atmosphere. The nitrogen atmosphere had less than 20 ppm oxygen and a dew point of less than -60.degree. C. (-76.degree. F.) and the hydrogen atmosphere had less than 10 ppm oxygen and a dew point of less than -60.degree. C. (-76.degree. F.). The strip temperature was cooled to about bath temperature and passed into the bath. An aluminum bath with silicon was stated to minimize the alloy layer and reduce brittleness. The method for preparing strip to be aluminized in a continuous coating furnace had a total treatment time of less than about 7 minutes.
Canadian patent application 2,071,189 coated chromium containing steel strip by using a method which included preannealing the strip, alkaline cleaning the strip, rinsing and drying the strip and radiantly heating the strip in a hydrogen-nitrogen (25-50% by volume hydrogen--balance nitrogen) atmosphere with substantially no oxygen and water vapor at a temperature below 1470.degree. F. (800.degree. C.) and typically 1350.degree. F. to 1400.degree. F. (733.degree. C. to 760.degree. C.) to limit the growth of chromium oxides. A controlled dew point of -30.degree. F. (-35.degree. C.) to -10.degree. F. (-23.degree. C.) at the entry side; -50.degree. F. (-45.degree. C.) to -45.degree. F. (-43.degree. C.) downstream; and -60.degree. F. (-51.degree. C.) in the snout was used to provide a reducing atmosphere for the chromium oxides. The strip was coated in an aluminum bath containing about 10% silicon. Preannealing the strip before it was subjected to the inventive method provided the same properties as annealing done on the coating line. There were no preannealing conditions given.
Prior coating methods for aluminizing chromium alloys without the use of additional coating layers have thus relied upon a coating furnace which cleaned the strip and annealed the strip in-line using hydrogen/nitrogen atmospheres with controlled levels of oxygen and dew points to avoid the oxidation of the chromium on the surfaces.
Bright annealing stainless steel in a protective atmosphere using a continuous annealing line or a box anneal has been done to prevent discoloration and provide a clean, bright surface condition. Pure hydrogen or a mixture of hydrogen and nitrogen are used to keep the surfaces in a bright condition. The material is used extensively for automotive trim, kitchenware and other applications which require a bright, shiny surface. The use of a hydrogen atmosphere is expensive and substituting nitrogen reduces the cost. Nitrogen, however must be controlled since it could lead to nitrogen pickup (nitriding) and hydrogen has the potential to cause hydrogen embrittlement. Box annealing practices have also been limited in the past in the control of dew point in the furnace required for producing a bright surface.
Another important consideration for any annealing practice is the condition of the steel surfaces after annealing. Most continuous annealing treatments include a pickling step to remove the scale on the surfaces. During heating, the steel may react with the oxidants such as oxygen, water and carbon dioxide to form oxides that make up scale. The annealing time, temperature and atmosphere will determine the nature of the scale. Chromium, aluminum, silicon and titanium on the surfaces are very easily oxidized.
The preparation of steel alloys containing chromium for hot dip aluminizing has been difficult in the past due to the poor wettability of the surfaces and the nature of chromium oxides. The present invention is directed to the production of preannealed chromium alloy steel surfaces for aluminizing and aluminized chromium alloy steel with greatly reduced uncoated spots.